1. Field of the Invention
The present invention relates to a driving mechanism for turning a photosensitive drum in an electrophotographic copying machine, which includes a brake loading means for braking the drum against a driving force for turning it.
2. prior Art
Generally, the electrophotographic copying machine produces an equivalent electrostatic latent image for an original image on a photosensitive material of the peripheral surface of the photosensitive drum by scanning the original image under exposure and then records a reproduced image on recording paper by making the latent image to be visible by a toner development and transfering the toner developed image to the paper.
Well, it is known in the art that it is very important to synchronize the scanning under exposure and the turning of the photosensitive drum, that is to synchronize the canning speed under exposure and the moving speed of the drum surface correspond1ng to a desired copying magnification.
However, since the driving mechanism for both scanning system and photosensitive drum system have power transmitting means generally composed of pulleys, belts, gears, etc. and also the forces of inertia of said both systems are different to each other, the synchronization therebetween becomes worse due to backlashes in the driving mechanisms.
Therefore, Japanese Utility Model Publication No. 31548 of 1980 discloses the driving mechanism of above mentioned type which is provided with a brake loading means for braking the turning of a photosensitive drum constantly by a predetermined frictional braking force. That is, by mounting the flange of the photosensitive drum rotationally to a fixed axis, fixing a spring retainer to the axis, then fitting a ring-shaped braking spring, which is axially deformable, loosely to the axis between the flange and the spring retainer and forcing this braking spring contact to the flange of the drum constantly under a predetermined pressure, it is intended for the brake loading means to apply the predetermined frictional braking force upon the turning of the drum and to synchronize the peripheral speed of the photosensitive drum (the moving speed of the drum surface) and the scanning speed under expouser.
However, since the brake loading means in the above described prior art is composed of the brake loading device only for the photosensitive drum, there exists a limitation for improving the synchronization.
For example, in FIG. 4, in case of producing an electrostatic image on the photosensitive drum 2 through the optical system 20 and transfering the toner developed image made visible by the toner development device 21 onto the recording paper 22 which is held in contact with the drum 2 by the action of the counter bias chager 23, as the feeding speed of the paper feeding system 26 is set a little slower than the moving speed of the drum surface to prevent the paper 22 from being slakened, the external force is applied to the drum in such a direction as making its turning speed slower. On the contrary, after the recording paper being removed from the photosensitive drum 2, the external force is applied to the drum in such a direction as making its turning speed faster. Therefore, as the adverse effect by the backlashes in the driving mechanism which connects the electric motor 3 to the photosensitive drum 2 is not eliminated by the above described brake loading means only for the drum, some distortions are generated in the part of the reproduced image correponding to the time when either the feeding or the removal of the recording paper is commenced, and consequently the production of the latent image and the transference of the former image onto the paper can not be done in parallel. In this connection, the flucture in the above mentioned speed of the drum in the prior art is around 0.2-0.3 % of its standard speed at that time. But it is necessary to keep the flucture within the extent of 0.1% in order to attain a reproduced image of high grade quality. The bad influences by the former flucture value can not be eliminated in the prior driving mechanism.
Although the braking force by the brake loading means can be increased as the countermeasure to eliminate the above described defects, it adds substantially to the cost because of increasing the out put power of the turning motor according thereto, hence, it is not advantageous. Further, even though it can be thought out that the electrostatic latent image is to be produced at the first turn of the photosensitive drum thereon and then the toner development and the transference of the toner developed image are to be done at the second turn of the drum, it has such defects as the time required for one cycle becomes two times and an image longer than one circuit of the peripheral length of the drum becomes impossible to be reproduced.